A flush joint tubular connection has inner and outer diameters substantially the same as the tubing joints which the connection connects. A flush joint tubular connection made by the Hydril Co., Inc., and covered by numerous patents comprise a first straight thread, a second straight thread of sufficient diameter to pass within the bore of the first thread and a tapered mating seal between the two joints of tubing which is a premium joint of high cost and according to published data, enjoys only 42% axial strength with regard to the pipe wall.
Standard A.P.I. non-upset tubing connections comprise couplings having outer diameters considerably larger than the pipe outer diameter but still only enjoy approximately 42% efficiency as above. A.P.I. does list a "turned down" collar outer diameter to increase clearance between strings, however, the "turned down" diameter still exceeds substantially, the pipe outer diameter.
No prior art discloses a tubular connection having tapered threads, that when properly assembled, effects optimum stresses within the small end of the external thread and within the large end of the internal thread so as to provide a connection of maximum efficiency. Conventional pipe connections have threads with like tapers and result in a constant diametrical interference along the taper between the external and internal threads, thereby causing excessive stresses or requiring increased wall thickness at the ends of the pipe joint. Excessive stresses reduce the joint strength and an increased wall thickness rules out a minimum diameter connection. It is therefore clear that a high efficiency tubular connection having a minimum diameter as provided by the instant invention is needed for use within oilwells and other pipe assemblies wherein radial clearance is limited.
Ameron Inc., Fiberglass Systems, Inc. and others now make RTR pipe, for use in the presence of corrosive fluids, according to A.P.I. Specification 5AR. The typical process comprises: moulding resin-wetted glass fibers around a cylindrical mandrel in a pattern designed to effect a desired combination of axial and cylindrical strengths; allowing the resin to harden; removing the joint from the mandrel; forming external threads on at least one end thereof in accord with A.P.I. Specification 5CT, the same thread specified for steel pipe. 5AR specifies the maximum diameter for tubing collars in Table 5.3 which are in part: 4.67" for 2.375 pipe; 5.52" for 2.875 pipe; 6.68" for 3.5 pipe; 7.57" for 4.5 pipe. These diameters are proportionally much greater than for like sizes steel pipe couplings: 3.06"; 3.67"; 4.5"; 5.56" respectfully, which together with the thicker pipe wall required, greatly complicates the radial clearance problem discussed above.
In an effort to escape the undesirable compressive characteristics of pin threads formed directly on RTR pipe, "premium connections" have been introduced which comprise; pin threads formed on two short pipes of selected plastic material better able to withstand compressive forces than can RTR material; splicing the short pipes to the ends of the RTR pipe by wrapping resin-wetted glass fibers around the ends of the members to be joined so as to form a pipe joint with pin threads at both ends. When such joints are joined by the use of collars, the compressive forces have been delt with but considerably more labor and material must be used and the new question of splice strength is now present and the large diameter is still a problem as previously discussed.
Pin threads are now formed on RTR pipe by machining or by moulding. Since RTR material is very difficult to machine, machined threads are rough and they have a great tendency to grab and tear out, thereby causing failure. Moulding pin threads on the RTR pipe requires subsequent forming such as the splicing described above or the application of moulds around the ends of the pipe joint after it is formed, which is considerably more complicated and less reliable than the moulding of box threads simultaneously with the forming of the pipe joint.